R6 Carbon battery.The impact of lithium iron battery formation process on battery performance

　　The impact of lithium iron battery formation process on battery performance. Regarding the formation of lithium iron phosphate batteries, activation is relatively difficult due to the low intrinsic conductivity of lithium iron phosphate. Therefore, it should be considered that before formation, the electrolyte should be fully infiltrated into the electrode, left at room temperature for more than 7 hours, and aged at high temperature (50-60) for more than 2 hours. It is best to consider low current and high voltage formation during formation.

　　The impact of lithium iron battery formation process on battery performance

　　Formation is an important process in the production process of lithium iron batteries. During the formation, a passivation layer is formed on the surface of the negative electrode, that is, the solid electrolyte interface film. The quality of the SEI film affects the cycle life, stability, and self-discharge of the battery. , safety and other electrochemical properties to meet the "maintenance-free" requirements of secondary battery sealing. However, the SEI films formed by different formation processes are different, and their impact on the performance of lithium batteries is also very different.

　　The traditional low-current precharging method helps to form a stable SEI film, but long-term low-current charging will increase the resistance of the formed SEI film, thereby affecting the rate discharge performance of lithium-ion batteries. The long process time affects production efficiency.

　　The formation process of lithium iron phosphate system is usually selected as follows

　　The charging current is 0.05C~0.2C, the cut-off voltage is 3.6~3.7V, and the charging cut-off current is 0.025C~0.05C. After standing for a period of time, discharge at 0.1~0.2C to 2.5V and let it stand for a period of time. Under different charging and discharging mechanisms, the difference in charging current affects the formation and quality of SEI, and the resting time and charging cut-off current affect the formation process time of the battery.

　　The battery formation process of the lithium iron phosphate system requires the selection of an appropriate cut-off voltage. In terms of the material crystal structure, when the charging voltage is greater than 3.7V, the lattice structure of the lithium iron phosphate may be damaged, thereby affecting the cycle performance of the battery. Some internal resistance experiments and pole piece SEM observation results also prove that the following conclusions are correct:

　　1. Appropriately reducing the formation voltage and formation time can effectively reduce the generation of lithium precipitation on the surface of the negative electrode, thereby obtaining a negative electrode piece with a smoother surface. This is because when the formation voltage is high, the gas production rate inside the battery is faster, so that the gas inside the battery cannot be discharged in time and is deposited on the surface of the separator, affecting the contact balance between the separator and the negative electrode. During the deintercalation process of lithium ions, the unbalanced contact between the two causes excessive intercalation of lithium ions in some areas, causing the surface of the negative electrode to be rough, and ultimately affecting battery performance.

　　2. After the internal resistance test of the lithium iron battery after formation, it was found that appropriately lowering the formation voltage and shortening the formation time can reduce the internal resistance of the battery. The high internal resistance caused by the high formation voltage is also related to the rough surface of the negative electrode and the formation of white spots. Because the white spots are lithium compounds and have poor conductivity, the internal resistance of the battery is relatively high.

　　3. Appropriately reducing the formation voltage in the design of the formation process can increase the first charge and discharge capacity of lithium batteries and improve the cycle performance of the battery. Excessive formation voltage can easily cause the deposition of lithium and its compounds on the surface of the negative electrode, which increases the irreversible capacity of the lithium battery and will inevitably affect the capacity of the battery. Due to the presence of lithium and its compounds, the capacity of the battery decreases during the charge and discharge cycles. The faster it affects the battery cycle life.

　　Lithium iron phosphate battery formation suggestions

　　1) Discharge capacity of positive electrode

　　2) First charge and discharge efficiency of the battery

　　3) Battery voltage platform

　　4) Constant current and constant voltage charging ratio of lithium battery

　　5) The difference between the charging and discharging voltage platforms of lithium batteries

　　6) The relationship between the capacity of 0.2c and 0.5c

　　Steps in the lithium battery formation process

　　The SEI film formation process during the lithium battery formation process specifically includes the following four steps:

　　Step ①: Electrons are transferred from the current collector-conducting agent-graphite particles to point A where the SEI film is to be formed;

　　Step ②: The solvated lithium ions, wrapped in the solvent, diffuse from the positive electrode to point B on the surface of the SEI film being formed;

　　Step ③: The electrons at point A diffuse to point B through the electron tunneling effect;

　　Step ④: The electrons that jump to point B react with lithium salt, solvated lithium ions, film-forming agents, etc., and continue to form an SEI film on the surface of the original SEI film, thereby increasing the thickness of the SEI film on the surface of the graphite particles, and finally forming a complete SEI. membrane.

　　It can be seen that the overall reaction process of SEI formation can be specifically decomposed into the above four step-by-step reactions to describe. The four step-by-step reaction processes determine the film-forming process of the entire SEI film.

　　The chemical process of lithium iron battery is to charge the battery for the first time, which activates the active materials in the battery and generates a dense film on the anode surface to protect the entire chemical interface. Formation is also called activation. After the lithium battery is manufactured, its internal positive and negative electrode materials are activated through a certain charging and discharging method to improve the overall performance of the battery. Formation is a very complex process, and it is also an important process that affects the performance of lithium batteries.

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